UW Shoulder and Elbow Academy: aspirate

Defining a Synovial Fluid White Blood Cell Count Threshold to Predict Periprosthetic Infection after Shoulder Arthroplasty

These authors sought to define a threshold for synovial fluid white blood cell count (WBC) and the reliability of microbiological cultures in predicting shoulder periprosthetic infection (PJI).

They conducted a retrospective study of preoperative and intraoperative fluid aspiration of 31 patients who underwent a revision of a shoulder arthroplasty (15 with PSI defined by the Infectious Diseases Society of America (IDSA) criteria (see this link) and 16 without infection).

WBC was significantly higher in patients with PSI than in other patients. A threshold of 2800 leucocytes/mm3 showed a sensitivity of 87% and a specificity of 88% (AUROC 0.92).

Microbiological cultures showed a sensitivity of 76% and a specificity of 100%. The observation that almost one quarter of the joint aspirate cultures were negative in cases of periprosthetic infection (PJI) indicates that positive cultures of joint fluid can "rule in" a PJI but a negative culture of joint fluid cannot "rule out" a PJI.

Interestingly, joint aspirate culture was less likely to grow Cutibacterium (23%) than the tissue cultures obtained at revision surgery (40%) (see below). There may be at least two reasons for this finding (1) the joint aspirate provides a single specimen whereas multiple (at least three) tissue samples increase the chances of recovering the organism and (2) Cutibacterium is more commonly present in a biofilm on tissue and implants than in a planktonic form where it would be accessible in a fluid sample.

Comment: Even though the number of cases in this study is small, the results are of interest and need to be confirmed in larger cohorts. This should be straightforward, in that white blood cell counts and cultures are easy to obtain and relatively inexpensive.

One issue not addressed in this study is the rate of insufficient sample volume, that is the percentage of cases coming for revision in which the amount of fluid obtained is not enough for both a WBC count and culture. How should this result be considered?

A second issue is that the utility of these tests should be examined in terms of the "pre-test probability". It is known that Cutibacterium PJI can present on one hand with systemic signs, increased blood markers (CRP, ESR, WBC) and on the other with only unexplained pain and stiffness. In the former the pre test probability of PJI is high and in the latter less so. How diagnostic are joint fluid aspirate WBC counts and cultures in these two groups?

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Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link)

Shoulder arthritis - x-ray appearance (see this link)

The smooth and move for irreparable cuff tears (see this link)

The total shoulder arthroplasty (see this link).

The ream and run technique is shown in this link.

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).

Arthroscopic Tissue Culture for the Evaluation of Periprosthetic Shoulder Infection

The senior authors evaluated approximately 350 painful shoulder arthroplasties performed during the study time period. Of these, 19 patients had undergone culture of arthroscopic biopsy tissue during the evaluation of a possible chronic periprosthetic shoulder infection. None of the patients in the study population had concomitant elevation of the WBC, ESR, and CRP values. The mean age at the time of the index arthroplasty was 56.7 years, and the mean interval (and standard deviation) between the index arthroplasty and the arthroscopic biopsy was 3.0 ± 2.0 years (range, 0.7 to 7.7 years)

Of these 19, 10 had negative arthroscopic biopsy cultures and negative open biopsy cultures taken at the time of surgical revision.

For those shoulders with positive cultures, the mean time for the culture to turn positive was 10.1 ± 3.79 days (range, 5 to 18 days).

7 had positive arthroscopic biopsy cultures for Propionibacterium and positive cultures for this same organism at the time of surgical revision.

2 had positive arthroscopic biopsy cultures for Propionibacterium but no cultures were taken at the time of revision.

6 shoulder joint fluid aspirations were done on patients who were subsequently found to have positive biopsy cultures for Propionibacterium - only one of the aspirates was culture positive.

The authors performed complete removal of the prosthetic components and placement of a custom antibiotic spacer in eight of the nine patients with a positive arthroscopic tissue biopsy culture at a mean of 11.2 ± 8.9 weeks (range, 3.4 to 29.3 weeks). The ninth patient had a single stage revision without apparent recurrence of infection.

Comment: This is a retrospective study of a small percentage of the authors' patients having revision surgery. The authors state that "Arthroscopy was performed in patients for whom the diagnosis of periprosthetic infection could not be reasonably confirmed or refuted by the patient history, physical examination, imaging studies, and laboratory data." One would think that there would be more than 19 of the 350 painful arthroplasties that met these criteria. The case the authors show in figure one is a common scenario: A patient with two years of pain and stiffness after a left TSA. Laboratory results were normal, and there was no growth from a fluoroscopically guided glenohumeral aspiration. An anteroposterior radiograph showed notable radiolucency about the pegged glenoid component". As prior recent posts have pointed out, glenoid or humeral component loosening is strongly suggestive of the presence of Propionibacterium. Again, since glenoid component loosening is a primary cause of failure of total shoulder arthroplasty, one might expect more cases like that shown in figure 1 among the 350.

It is of interest that the time between between the index arthroplasty and the diagnosis of positive cultures was long: 3.0 ± 2.0 years (range, 0.7 to 7.7 years). This is consistent with the results discussed in a prior post.

Perhaps a bigger question is the protocol to be used in evaluating shoulders for possible Propionibacterium and for treating shoulders with positive cultures. The protocol one might conclude from this paper is that, since as their data show, the presence of Propionibacterium is not at all excluded by the absence of clinical and laboratory evidence of infection, all revisions should be preceded by an arthroscopic biopsy under anesthesia. If the culture results are negative, then a single stage revision is performed. If the culture results are positive a two stage revision is performed. This means that revision of a failed shoulder arthroplasty would commonly require two or three surgical procedures.

By contrast in our practice (as summarized here), we try to reduce the number of anesthetics to one by obtaining cultures at the time of the revision, performing a single stage prosthesis revision to an uncemented Vancomycin-impregnated impaction grafted hemiarthroplasty followed by systemic antibiotic treatment until the culture results are finalized (at which time the antibiotic program is modified by our infectious disease consultations). We have yet to find convincing data to support the routine use of cement spacers in the management of Propionibacterium-positive failed arthroplasties.

Finally, as shown in this paper, culture of needle aspirated joint fluid is helpful only if the culture is positive, but not it it's negative.

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